Miniature electron beam microcolumns ("microcolumns") are based on microfabricated electron "optical" components and field emission sources operating under principles similar to scanning tunneling microscope ("STM") aided alignment principles. Field emission sources are bright electron sources that are very small, making them ideal for use in microcolumns. One type of field emission source is a Schottky emitter, such as the type discussed in "Miniature Schottky Electron Source," H. S. Kim et al., Journal of Vacuum Science Technology Bulletin 13(6), pp. 2468-72, November/December 1995 incorporated herein by reference. For additional field emission sources and for information relating to microcolumns in general, see the following publications and patents: "Experimental Evaluation of a 20.times.20 mm Footprint Microcolumn," by E. Kratschmer et al., Journal of Vacuum Science Technology Bulletin 14(6), pp. 3792-96, November/December 1996; "Electron Beam Technology-SEM to Microcolumn," by T. H. P. Chang et al., Microelectronic Engineering 32, pp. 113-130, 1996; "Electron-Beam Microcolumns for Lithography and Related Applications," by T. H. P. Chang et al., Journal of Vacuum Science Technology Bulletin 14(6), pp. 3774-81, November/December 1996; "Electron Beam Microcolumn Technology And Applications," by T. H. P. Chang et al., Electron-Beam Sources and Charged-Particle Optics, SPIE Vol. 2522, pp. 4-12, 1995; "Lens and Deflector Design for Microcolumns," by M. G. R. Thomson and T. H. P. Chang, Journal of Vacuum Science Technology Bulletin 13(6), pp. 2445-49, November/December 1995; U.S. Pat. No. 5,122,663 to Chang et al.; and U.S. Pat. No. 5,155,412 to Chang et al., all of which are incorporated herein by reference.
FIG. 1 shows a schematic cross sectional view of a conventional field emission source 10, which includes an electron emitter 12 and an extraction electrode 14. The electron emitter 12 is a Schottky emitter with a tungsten tip 16 serving as a cathode, which is spot welded on a filament 18. The filament 18 is mounted on two rods 20, which are held by a base 22, and is surrounded by a suppressor cap 24.
The extraction electrode 14 defines a center aperture 15. The aperture 15 and following (downstream) lenses (not shown) in the microcolumn define the optical axis 26 for the field emission source 10.
By applying a voltage Vc to the tip 16 and a voltage Ve to the extraction electrode 14, a resulting electric field causes the emission of electrons from tip 16. A voltage Vs applied to the suppressor cap 24 suppresses undesired thermionic electrons.
An important consideration in the field emission source 10 is that the electron emitter 12 is aligned with the optical axis 26. Because the diameter of aperture 15 is typically 1-2 .mu.m (micrometers), a small misalignment, e.g., 1 .mu.m, will result in a large off-axis aberration and an undesirable increase in the total spot size. Thus, a small misalignment can severely deteriorate the performance of a microcolumn.
Conventionally, to ensure proper alignment, the electron emitter 12 is mechanically aligned with the optical axis 26. Thus, electron emitter 12 is physically moved, as indicated by arrows 28, by the use of, e.g., alignment screws, a micrometer x-y stage, a piezoelectric stage, or a scanning tunneling microscope (STM) like positioner to align position electron emitter 12 with optical axis 26. Unfortunately, mechanical alignment is difficult to achieve and is difficult to maintain over extended periods of time due to drift problems.
Thus, there is a need for a field emission source that can be easily aligned with the optical axis.